Infectious bursal disease virus (IBDV) belongs to the Birnaviridae family. It causes a highly contagious immunosuppressive disease in chickens by depleting B cell populations within the Bursa of Fabricius.
The IBDV virion consists of a double stranded RNA genome in a non-enveloped icosahedral capsid. The capsid contains two major structural proteins, VP3 and VP2. Virus neutralizing antibodies have been shown to recognise a conformational epitope located in the hypervariable region of VP2, between amino acid residues 206 and 350. Thus, the VP2 protein is an important target for immunodetection and immunoprophylaxis studies.
A large number of IBDV strains have been identified throughout the world and grouped according to their relative pathogenicity and antigenicity. Three major groups have been identified: (i) classical strains, (ii) very virulent types (vvIBDV) capable of causing up to 70% mortality and (iii) antigenic variants. In Australia both classical and variant field strains of IBDV have been identified which are genetically distinct from overseas strains. Australian classical strains are similar to vaccine strains such as 002/73 and V877 both antigenically and by nucleotide sequencing. Australian variants, however, are quite distinct both at the antigenic and genetic level forming a distinct group of viruses that are unrelated to variants isolated in other countries such as the USA.
The ability to differentiate and treat infection caused by IBDV strains is a major concern to the poultry industry worldwide, as effective control of this devastating disease relies on the administration of different vaccine strains. In many countries both accidental incursion of IBDV strains from neighbouring countries and the absence of a test for establishing whether outbreaks in the field are due to vaccine breaks or circulating field strains are also major problems.
Differentiation of IBDV strains is currently largely based upon using either monoclonal antibodies directed against the major neutralising protein of the virus (VP2) or by nucleotide sequencing of the VP2 gene. Monoclonal antibodies generated against the VP2 and VP3 gene product have perhaps played the most significant role in detecting and differentiating IBDV strains and several VP2 and VP3 specific monoclonal antibodies have been developed.
Testing with monoclonal antibodies is generally quicker and significantly cheaper than other prior art technologies, however, monoclonal antibodies are limited in terms of their differentiation. Furthermore, a major drawback of using monoclonal antibodies as immunodiagnostics or immunotherapeutic reagents lies in the cost and time required for production, screening and maintenance of hybridoma cell lines. In addition, an anti-antibody response is often induced when monoclonal antibodies are administered in a heterologous species, thus compromising their effectiveness.
Thus, there is currently a need for improved diagnostic and or therapeutic agents effective against IBDV. In particular there is a need for diagnostics that are capable of differentiating IBDV strains, that might be used to track the incursion of exotic strains of IBDV into a particular country and or for differentiating IBDV strains very similar to vaccine IBDV strains from circulating field strains. In addition, there is a need to differentiate between an immune response induced by IBDV vaccines from that induced by field strains. There is also strong demand for new therapeutics capable of combating this devastating disease.